For many years it has been the practice in the manufacture of ring binders to fabricate the cover for such binders in a three-ply construction. Three rigid or semi-rigid, rectangular inserts or stiffener panels are heat-sealed between two superimposed sheets of thermoplastic material. Of the three stiffener panels generally used, two of them approximate in size, the back and front cover panels of the binder and the third panel is a narrower insert strip disposed between the two larger panels to form the back panel of the binder. The inner and outer plastic sheets are fused together or heat-sealed around their peripheral edges and also transversely between the adjacent, transverse edges of the cover panel inserts and the back panel inserts. The transverse seals form the hinge areas of the binder and U.S. Pat. No. 3,195,924 is typical of this type of binder construction.
There are several significant disadvantages of a conventional heat-sealed binder cover, particularly along the hinge portions where the cover and the spine or backbone of the binder intersect. Along the hinge lines, the outer plastic sheets are fused or heat-sealed together and their composite thickness is substantially less than the total thickness of the two sheets before the heat-sealing operation. For example: where the two outer sheets of thermoplastic are each 0.015 inch vinyl, the composite thickness, when fused together, is only about 0.020 inch or about 30% less than the total of 0.030 inch where there has been no heat-sealing of the plastic sheets.
In addition, during heat-sealing, the more volatile plasticizers in the vinyl sheet materials are volatilized "off" and the hinge lines tend to be more brittle than the unfused vinyl. Moreover, during the heat-sealing operation, the vinyl films along the hinge lines are invariably stretched over the edges of the chipboard inserts when drawn together for sealing so that the plastic sheet material in these areas becomes thinner than the unsealed vinyl and thus more susceptible to material fatigue failure.
The principal object of this invention is to provide an improved ring binder construction and method of fabricating the same which overcome the drawbacks of the prior art construction.
Another object of this invention is to provide an improved ring binder cover having a backbone and hinge construction of remarkably enhanced durability without sacrifice in either the appearance or functionality of the binder.